1. Field of the Invention
This invention relates to a technology for reducing core loss in grain-oriented electrical steel sheet used for a static inductometer such as a transformer.
2. Description of the Related Art
Grain-oriented electrical steel sheet is mainly used for a static inductometer such as an electrical transformer. Required properties for the grain-oriented electrical steel sheets are: (1) low power loss during magnetization in an alternating current field, i.e. low core loss, (2) permeability is high in the range for the induction used for the machinery and devices and the sheet is easily magnetized, and (3) magnetostriction which causes noise is small. Especially, the requirement (1) is one of the most important factors of the transformer for evaluating its T.O.C. (Total Owning Cost) which is a measure of cost performance of transformer, since the power loss over time for a transformer continues for a long period from its installation to scrapping.
In order to reduce the core loss of grain-oriented electrical steel sheets, there have been developed many improvements such as: (1) increasing the intensity of (110)[001] orientation which is known as the Goss orientation, (2) increasing the content of solid solution elements such as Si which increases the electrical resistance, (3) decreasing the sheet thickness, (4) coating a ceramic film or insulation film onto the sheet in order to impart surface tension thereto, and (5) decreasing the size of grains. There are, however, limitations in the improvements by these metallurgical approaches, and other ways of reducing core loss have been sought.
As disclosed in U.S. Pat. No. 3,647,575, to Fieldler et al., a method was proposed to reduce core loss by refining the magnetic domains by imparting grooves to the surface of electrical steel sheets with a cutter. The grain-oriented electrical steel sheets have rectangular slab-shaped magnetic domains each of which adjoins another domain having the opposite magnetic polarity (hereinafter the magnetic domain is referred to simply as “domain”). A grain-oriented electrical steel sheet is magnetized as a result of expansion or shrinkage of each of the domains caused by an applied magnetic field. Thus, when the grain-oriented electrical steel sheet is magnetized, magnetization changes occur only at the neighborhood of the domain boundaries (domain walls) between adjoining domains. With this change, an eddy current generates in the steel sheet to cause eddy current loss which accounts for 60% to 70% of the core loss. The eddy current loss is in proportion to the square of the eddy current and is also in proportion to the movement speed of the domain wall. If each of the domains is refined to a small size, the number of portions where the eddy current occurs increases. However, since the movement speed of the domain wall decreases in inverse proportion to the width of the domain, the eddy current loss as a whole decreases almost in proportion to the width of the domain.
Various inventions are proposed in order to make this domain refinement technology industrially applicable. JP S58-5968B discloses a method to provide strained portions without making scratches on the surface of an electrical steel sheet by pressing and rolling small balls of 0.2 to 10 mm in diameter on the surface of an electrical steel sheet. JP S57-2252B discloses a method to provide an electrical steel sheet with small plastic strained portions by irradiating a laser beam on the surface of an electrical steel sheet in the transverse direction. JP S62-96617A discloses a method to provide an electrical steel sheet with small plastic strains by focusing plasma flame on the surface of the electrical steel sheet lineally in the transverse direction. These methods are based on domain refining technology which utilizes a domain which is stabilized with magnetized component in the direction perpendicular to the rolling direction, as a result of reciprocal effect (reverse mechanism) of magnetostriction, by small plastic strains introduced onto the electrical steel sheet. Especially, the grain-oriented electrical steel sheet of which domain is refined with laser irradiation (hereinafter referred to as “laser-domain-refined grain-oriented electrical steel sheet”) is widely used in industry for a large-sized lamination type power transformer for which low core loss is required. The demands for such electrical steel sheets have increased tremendously in recent years because of the global trends for the reduction of energy consumption aiming at CO2 output reduction.
However, the technology disclosed in the above-mentioned JP S58-5968B uses only mechanical strain, which may fail to bring a great reduction of core loss and make it difficult to apply the technology industrially because the technology requires pressing and rolling small balls in the transverse direction. The technology disclosed in the above-mentioned JP S57-2252B can reduce core loss well, however, further improvement is required for reducing magnetostriction. In the technology disclosed in above-mentioned JP S62-96617A, it is difficult to control the amount of strain, and thus there remains a problem in obtaining stably reduced core loss.
JP 2647322B discloses a method to produce a low core loss grain-oriented electrical steel sheet where an electrical steel sheet is melted by laser beam irradiation in the form of a line and re-solidified, the re-solidified portion is 50 to 300 μm in width, 5 to 35% of the sheet thickness in depth and is located in a direction ranging within ±15 degrees from the direction perpendicular to the rolling direction, and the interval between adjoining lines is 5 to 30 mm. Then the sheet is finally coated with an insulating film for imparting tension. This technology, however, is intended for the use in small wound-core type transformers which are stress relief annealed, and if the technology is used in large sheared-flat-lamination type transformers which are not stress relief annealed, the introduced excess strain makes it difficult to stably obtain an electrical steel sheet having the properties of low core loss as well as low magnetostriction and may even reduce the core loss.
Static inductometers such as transformers and reactors make noise when the core is magnetized by an alternating current field. The noise reduction is strongly demanded, since the number of transformers installed in urban areas is increasing according to the increase in demand for electricity. In addition, there is the trend of minimizing environmental impact. The noise is usually caused by the following; i.e., the vibration between the induction coils induced by electromagnetic force, the vibration in the joints of the core and between laminations induced by magnetic force, and the vibration of magnetostriction of electrical steel sheets.
Among these sources of noise, the noise from the core material can be reduced by the methods described below, for example: (1) the core is designed to work at a lower magnetic flux density because of a lower magnetostriction at low magnetic flux density of the electrical steel; (2) highly grain-oriented electrical steel is used so as to reduce the magnetostriction and the tension of the surface coating film on the electrical steel sheet is increased as disclosed in IEEE Transaction, MAG-8 (1972), p. 677-681, “Magnetic Properties of Grain-Oriented Silicon Steel with High Permeability Oriented Core HI-B”, T. Yamamoto et al.; (3) specific arrangements for uniformly pinching the core is made as described in JP S47-28419A; (4) the core is covered with a sound isolation box as described in JP S48-83329A; and/or (5) the transformer is placed on a rubber cushion as described in JP S56-40123A.
These methods, however, are very costly because these methods require extra equipment to be added to transformer.
As reported in the Journal Of The Magnetic Society of Japan, Vol. 25, No. 4-2, 2001, the property of the magnetostriction with “laser-domain-refined grain-oriented electrical steel sheet” varies according to the conditions used for laser irradiation. More specifically, the property of the magnetostriction varies as the irradiation energy density of the laser Ua varies, therefore, the magnetostriction of the electrical steel sheet can be reduced by selecting relevant Ua value. With the method mentioned above, however, it is difficult to obtain the maximum effects regarding the reduction of magnetostriction.
As mentioned above, although the grain-oriented electrical steel sheet has been greatly improved with respect to core loss, still further improvement is demanded in view of increased energy consumption, increased concern about the drain of fossil energy and the requirement for countermeasures of global warming. As for the noise generated by the transformer, reduction of the noise has been further demanded as transformer installation is made in urban area.